Two uncharged metal plates in a total vacuum still pull toward each other

In 1948, Hendrik Casimir predicted that two uncharged, perfectly conducting plates held extremely close together in a vacuum would attract one another, caused by a shift in the quantum vacuum's fluctuating energy between the plates versus outside them. Nearly fifty years later, physicist Steve Lamoreaux measured the force directly and found it matched Casimir's prediction to within about 5%. It's now a real nuisance in microscopic machines, where it can make tiny components stick together.

— Hendrik Casimir, On the Attraction Between Two Perfectly Conducting Plates — Proceedings of the Royal Netherlands Academy of Arts and Sciences, 1948; confirmed by Steve K. Lamoreaux, 1997